Conventionally with electronically controlled fuel injection units for internal combustion engines, a known method for making a fuel injection quantity per unit time of a solenoid type fuel injection valve constant, involves adjusting the fuel pressure supplied to the fuel injection valve using a pressure regulator, and metering the quantity of fuel injected into the engine using a pulse width (a valve open control period of the injection valve) of an injection pulse signal fed to the fuel injection valve.
The pressure regulator is in general disposed in the fuel piping close to the fuel injection valve, and adjusts the fuel quantity returning to the fuel tank by way of a return path, so as to maintain a constant differential pressure between the fuel pressure and the pressure at the injection orifice of the fuel injection valve (engine boost pressure).
In the case where the pressure regulator is disposed in the vicinity of the engine, then the fuel which is returned to the fuel tank for fuel pressure adjustment by the pressure regulator, absorbs heat from the engine, thus causing the temperature inside the fuel tank to rise.
Accordingly a system has been developed wherein the pressure regulator is located inside the fuel tank, so that the fuel is circulated within the fuel tank, thus avoiding a temperature rise therein due to the fuel returning from the pressure regulator.
However, with this construction also where the pressure regulator is provided inside the fuel tank, in order to control the fuel pressure to a predetermined value, it is necessary to take out the pressure of the injection orifice of the injection valve (engine boost pressure), as a reference pressure for the pressure regulator. Hence the long negative pressure piping must be installed for taking the engine boost pressure to the pressure regulator inside the tank.
There is thus the situation where, if the pressure regulator is housed inside the fuel tank, the temperature rise inside the fuel tank can be avoided, and there is no requirement for a long return path for returning the fuel from the pressure regulator to the tank. However, there is instead the requirement for long piping for taking out the boost pressure as a reference pressure, resulting in a deterioration in response, and no real improvement from the point of view of piping construction.
A device has therefore been proposed (refer to Unexamined Japanese Patent Publication No. 64-73133) wherein the negative pressure line is abolished, and the pressure adjustment chamber of the pressure regulator is opened to the atmosphere, thus making the reference pressure atmospheric pressure. Since in this case the fuel injection pressure is adjusted to give a constant differential pressure with the atmospheric pressure as the reference pressure, the differential pressure for the injection orifice is no longer constant. The fuel pressure (or the atmospheric pressure), and the intake air negative pressure (boost pressure) are therefore measured using pressure sensors, and the injection pulse width (fuel injection period) corrected based on the differential pressure therebetween.
However, with such a method using two pressure sensors, the cost is increased.